Two‐Dimensional Porosity of Crusted Silty Soils: Indicators of Soil Quality in Semiarid Rangelands?

Abstract

Little is known about the morphological characteristics of pores in soil crusts. The objective was to characterize the two‐dimensional (2D) porosity (amount, shape, size, and area of pores) of soil crusts to ascertain their potential as indicators of soil quality for natural crusted soils. The 2D‐porosity was described in thin sections and measured by image analysis of polished resin‐impregnated soil blocks. Physical soil crust and incipient biological soil crusts appear to be the lowest‐quality soils in terms of number of pores (average of 131–133 cm−1) and area occupied by pores or meso‐macroporosity (3.5–4.2%). Their most abundant pore types were small unconnected rounded pores. Soil crust infiltration coefficients (65–72% annual) were among the lowest and their high erosion rates (81–204 g m−2 yr−1) were not only due to their lower total porosity, but also to their pore shapes and sizes. Biological soil crusts appear on higher‐quality soil, where the higher the organic C content, the more evolved the soil crust is (with lichens and cyanobacteria). Such soil crusts have better developed pore‐systems with specific meso‐ and macropore morphologies, for example, large, interconnected elongated and irregular pores. Biological soil crusts (BSCs) dominated by lichens have the largest meso‐macroporosity (up to 23.65%) due to the predominance of elongated pores. In many cases, infiltration is low (46–57%) because the biological crusts are somewhat detached from the soil underneath, but the armouring effect decreases erosion rates (7–23 g m−2 yr−1). Soil crust pore numbers, size, and shape were useful indicators of soil quality.